The present invention relates to a sliding ring non-return valve. In particular, the valve is used to prevent reverse material flow therethrough in a consistent and repeatable manner while minimizing the potentially damaging friction of the ring against its downstream stop and the surrounding barrel.
Injection moldingxe2x80x94the process of injecting a quantity, or shot, of molten plastic into a moldxe2x80x94is today one of the world""s dominant forms of plastic article manufacture. However, a product uniformity problem plagues this process because of the inability to control perfectly the quantity of material injected into the mold. This imperfection is caused primarily by the failure of a non-return valve, found on most injection molding machines, to close in a consistent, repeatable manner during the injection step.
Non-return valves allow molten plastic to flow from the feed screw to an accumulation volume. Injection occurs when the accumulation volume is full. During injection, the entire valve assembly strokes forward, forcing the molten material from the accumulation volume into the mold. Non-return valves are designed to shut during the injection process, preventing back flow through the valve. A review of the prior art illustrates two primary methods of sealing against material back flow during the injection step: a ring-type shut-off valve or a ball-type check valve. With these methods, as the injection ram strokes forward, a ball or piston is forced against a seat or a tapered ring is forced against another ring with a complementary taper. With a ball-type valve, plastic back leakage over the ball creates a pressure drop across the ball. This pressure drop becomes the primary force closing the valve. However, any back leakage variation before the valve closes causes a variation in the quantity of plastic in the accumulation volume.
Ring-type valves use a cylindrical ring rather than a ball to close the valve. FIGS. 1a and 1b illustrate an example of a prior art ring valve. This ring fits outside the valve body, through which the molten material flows. A barrel contains the entire assembly. The ring""s upstream and downstream travel are limited by upstream and downstream retainers. The ring""s upstream face is tapered, or angled a certain amount from perpendicular to the valve""s longitudinal axis. When open (FIG. 1a), material flows over the ring""s upstream stop, between the upstream face and the upstream retainer, under the ring through grooves in the valve body, and out of the valve through grooves in the downstream stop. When closed (FIG. 1b), the ring""s upstream face""s tapered surface contacts the upstream stop""s complimentary surface, forming a sealing surface and blocking flow through the valve. Thus, when the ring is in its downstream position, the valve""s flowpath is open; when upstream, the ring blocks the flowpath. Existing ring valves require a land length for sealing. The greater the length of this sealing surface, or land length, the better the seal. The ring/valve body assembly is attached to the downstream end of a feed screw. As the screw rotates, it feeds material into the non-return valve. The material forces the ring to its open position, allowing the material to flow through the valve and into an accumulation volume.
Ring valves do not rely upon a back leakage-induced pressure drop to close the valve. Instead, ring valves rely upon a very tight clearance between the ring""s outer diameter and the barrel""s inner diameter to create a friction force that holds the ring in place as the valve body strokes forward during injection, forcing the shot of plastic out of the valve and into the mold. In other words, when the valve body strokes forward, the friction force tends to hold the ring in place relative to the stationary barrel. As the valve moves forward within the barrel, the ring moves upstream relative to the valve body, blocking the material flowpath. The friction force between the ring and the barrel also opposes the ring""s downstream travel when the screw begins to rotate and feeds material into the valve body. This reduces the ring""s downstream velocity relative to the valve body and the resultant force with which it impacts the downstream retainer.
The friction between the ring and the barrel also creates a force opposing the rotation of the ring along with the valve body when the screw rotates and feeds material through the valve body. Thus, the valve body tends to rotate inside the ring, which is held almost stationary against the barrel""s inner surface by the friction force. Because the valve""s downstream retainer rotates with the valve body, and the ring""s downstream surface contacts the downstream retainer, a rotational friction force results between the ring""s downstream seating surface and the retainer. This results in wear of the ringxe2x80x94shortening the ringxe2x80x94and of the retainer. The resulting erosion of these surfaces increases the distance that the ring must travel upstream during injection to cover the valve inlets, increasing valve closure time and valve leakage prior to closing. As cumulative wear between the ring""s outer surface and the barrel""s inner surface increases the clearance between the ring and the barrel, the friction force decreases and dynamic forces created by the material flow become the primary forces shutting the valve. However, these same flow-induced dynamic forces can cause the ring to open too quickly and impact with great velocity upon the ring""s downstream retaining device. Moreover, the decreased friction between the ring and barrel allows the ring to impact the downstream retainer with greater velocity. Finally, while a longer land length, or sealing surface, minimizes leakage through the valve, the increased surface area of the ring""s upstream surface results in a greater force (for a given pressure) upon the ring in the downstream direction when flow-induced dynamic forces close the valve. Over the valve""s life, this impact can cause increased wear and even premature valve failure. To minimize ring and downstream stop erosion, prior art valves are often necessarily constructed of very hard materials, such as H-13 tool steel, with specially hardened retaining surfaces. These materials are expensive; moreover, their increased brittleness increases the chances of catastrophic failure due to repeated impact.
With both ball and ring type valves, particulate contamination, poor alignment, and wear of the sealing surface worsens back leakagexe2x80x94and consequent injection mass variationxe2x80x94by preventing a perfect seal and allowing back leakage through the valve even after valve closure. Therefore, a need exists for a non-return valve that always furnishes the same shot size regardless of plastic, fillers, contamination, product produced, or wear. This valve should be designed to allow its incorporation into existing injection molding machines or any other device which utilizes a non-return valve. This valve should not depend solely upon leakage through the valve or barrel to ring friction to generate the force necessary to move the valve to its closed position. Furthermore, this valve should be designed so that particles can never impair the seal. Finally, the valve should limit the impact of the closing ring against its retaining devices.
The present invention is a sliding ring non-return valve designed for use in an injection molding device with a screw-type injection plunger; however, it may be used in any application requiring one-way liquid or gaseous material flow. In its simplest form, the valve comprises a frame having one or more feed grooves along its outer surface through which the material flows. These grooves may be parallel to the valve""s longitudinal axis, but they may be any shape, (helical, for example) as long as they lead from the inlet groove to the accumulation volume at the valve""s downstream end. The frame is generally cylindrical and typically has a tapered, conical retaining cap integral to the distalxe2x80x94or downstreamxe2x80x94end. The proximalxe2x80x94or upstreamxe2x80x94end is threaded to allow for connection to a screw located in a barrel. The valve""s outer diameter must closely approximate the barrel""s inner diameter. An inlet groove, cut into the frame in a plane perpendicular to the frame""s longitudinal axis allows material to flow into the feed grooves. Because material flows through the valve, its proximal end is also referred to as xe2x80x9cupstreamxe2x80x9d and its distal end is also referred to as xe2x80x9cdownstream.xe2x80x9d
A ring is dimensioned to fit outside the frame. Unlike existing ring valves, the present invention does not require land length for proper sealing; therefore, the ring may be much thinner than those found on existing ring valves. The clearance between the ring""s outer diameter and the barrel""s inner diameter is normally greater than existing ring valves. However, leakage between the ring""s outer surface and the barrel""s inner surfacexe2x80x94or external leakagexe2x80x94is prevented both by the clearance and the ring""s length. When design considerations require a shorter valve length, ring length may be limited. Thus, a closer clearance may be required to minimize external leakage. Nevertheless, the clearance normally will be greater than the clearances found on existing ring valves. This increased clearance decreases the friction force between the ring and barrel and allows the ring to slide more easily. The retaining capxe2x80x94an annular cap integral to the valve""s downstream endxe2x80x94limits the ring""s downstream travel. A flange surface, or disk, with a diameter less than that of the barrel""s inner diameter, limits the ring""s upstream travel. The flange surface slidably attaches to the frame""s upstream end with the feed screw holding the flange in position. When the ring is in an upstream position, the ring blocks the inlet groove; when in a downstream position, the ring partially uncovers this groove.
The screw feeds material, typically molten plastic, through the gap between the flange surface""s outer diameter and the barrel""s inner diameter. This material reaches the feed grooves via the inlet groove. The material forces the ring towards its downstream position, partially exposing the inlet groove. The valve is designed such that the ring uncovers the minimum area necessary to assure adequate flow into the longitudinal grooves. In effect, the ring, because it only partially opens the inlet groove, throttles material flow. However, because the fluid has to travel only a short distance (essentially, only the edge or chamfer on the ring""s lower upstream corner) to the inlets, and the fluid is typically a compressible, elastic fluid such as molten plastic or resin, the pressure drop across the inlet groove opening is minimized.
The difference between the cross-sectional area of the flange surface and the barrel is the area through which the material enters the valve from the feed screw. The diameter of the flange surface is designed to limit this areaxe2x80x94and, consequently, the flowratexe2x80x94decreasing the resulting force against the ring""s upstream surface and the ring""s velocity in the downstream direction. The ring stops against the retaining cap. The material proceeds into the inlet groove and the feed grooves, out of the valve, and into an accumulation volume. When this area collects a selected amount of plastic, the screw stops rotating. Typically, the back pressure, used to assist the screw in melting plastic, is reduced to a minimum. To prevent xe2x80x9cdrooling,xe2x80x9d or plastic leakage out of the injection nozzle before injection, the valve and the screw are translated upstream a short, fixed distance. This is called xe2x80x9cpullback.xe2x80x9d
At this point, the mold already contains a quantity of plastic that is cooling. After this previously-produced part cools sufficiently, the mold is opened, the part removed, and the mold again closed. The screw ram then moves forward in the barrel, increasing pressure in the accumulation volume and forcing material into the mold. This pressure tends to push material back into the outlet passage and back through the valve. However, as the screw ram moves forward, it forces material back upstream around the periphery of the retaining cap, pushing the ring to its rearward closed position and blocking material flow into the inlet groove.
The volume of the injected shot is determined by the distance the screw and the valve are displaced in the upstream direction before closure. As material fills the accumulation volume, the screw is displaced rearward away from the accumulation volume. This rearward movement is limited to an exact position corresponding to a given volume of plastic to be injected. When the screw reaches this set position, it stops. No more material is added to the shot. Pullback affects the pressure but the mass remains constant for injection.
Depending upon the material and pressure involved, the valve may shut automatically when the screw stops turning and before pullback. This is called xe2x80x9cpreclosure,xe2x80x9d and it occurs as follows: As the feed screw rotates and feeds material through the valve and into the accumulation volume, the pressure in the accumulation volume increases as described above. As the accumulation pressure increases, certain fluidsxe2x80x94such as molten plastic and resinxe2x80x94compress. When the screw stops turning and the accumulation volume pressure relaxes, the compressed material expands, flowing around the outside of the downstream retaining cap and applying a force to the ring""s downstream seating surface, forcing it upstream to cover the inlet groove and closing the valve. Preclosure further minimizesxe2x80x94and may in fact eliminate completelyxe2x80x94valve leakage during the injection step by closing the valve before the screw ram moves forward. Pullback tends to open the valve by applying a negative pressure on the ring""s downstream face, forcing the ring downstream a small distance. However, this distance is so small that the valve opening will not typically be exposed after pullback.
In an alternative preferred embodiment, the material flows through the valve""s body rather than through grooves cut along the valve body""s outer surface. Inlet holes lead from the frame""s outer surface to the inner bore of the frame. When the valve is in its closed position, the ring covers the inlets. When open, the ring uncovers these openings. A retaining cap limits the ring""s downstream travel. The cap may be threadably attached to the frame""s downstream end or may be integral to the frame. A flange surface, which may be integral or removably attached to the frame, attaches to the valve frame just upstream of the inlet holes and downstream of the screw, limits the ring""s upstream travel. The flange surface has one or more grooves cut into its periphery. Material flows from the feed screw into the valve""s inlets through these grooves. The dimensions of these grooves are such that the combined cross sectional area of the grooves approximates the combined cross sectional areas of the inlet holes. Other than these design differences, the alternative embodiment operates in essentially the same manner as the above-described design.
Unlike prior art sliding ring and ball check valves, closing either embodiment does not require undesired leakage flow across the valve. Thus, the ring closes quickly and repeatably at the start of the injection step. The ring will also minimize clogging and leakage after closure because it will shear away any contaminants in its way. Finally, because the disclosed valve does not rely on a tight clearance between the ring and the barrel for proper operation, erosion due to friction is less of a concern. Thus, the valve may be constructed of less expensive, less exotic, softer materials.